Secondary usage of cellular spectrum by cognitive radio devices (CgNs) takes advantage of spectrum that would otherwise be unused by normal cellular users currently present in a cell. The cognitive usage of cellular spectrum is referred as secondary and the traditional usage is referred as primary. The secondary use problem is one of opportunistic use of available spectrum. One way this may be accomplished is passively: a secondary WTRU may observe that the spectrum is unused and may utilize the opportunity for its own communication. A second way to accomplish this is an active one: a secondary WTRU may act to create a spectrum opportunity for itself.
In the downlink transmission, the communication is under full control of the Node B (i.e., more broadly the infrastructure) and thus a secondary WTRU does not have to act on its own initiative. Transmissions to the WTRU may be scheduled in a manner consistent with its role as a secondary user (i.e., at a low priority). However, there exist opportunities for secondary mobiles to take advantages of the under utilized spectrum allocated to primary users in the uplink.
For example, the secondary usage of cellular spectrum may be transparent to primary users. The primary users may observe no degradation of service, and in the case of an actively cooperating secondary WTRU, a primary WTRU may experience a slight improvement through a reduction in latency. Accordingly, there is a need to provide a system that enables secondary usage of licensed cellular spectrum.
Background on existing cellular system is provided herein. For notational convenience the primary WTRU is referred to as a mobile station (MS) or primary WTRU. The secondary WTRU is referred to as the cognitive node (CgN) or secondary WTRU.
FIG. 1 illustrates the basic uplink traffic signaling in Long-Term Evolution (LTE) as shown in the message sequence diagram. The signal sequence along with the physical channel on which the signal is sent is shown in FIG. 1.
LTE considers three main types of uplink resource scheduling for assigning resources to a WTRU.
In dynamic scheduling, uplink (UL) resources are granted for one transmission at a time. Therefore, a WTRU may repeat scheduling requests to continue to receive UL grants to transmit data. This method provides dynamic resource allocation based on instantaneous channel conditions, however, the frequent grant requests adds a lot of signaling overhead which affects quality of service (QoS) for real time services.
Semi-persistent scheduling is a combination of dynamic and persistent scheduling. With semi-persistent scheduling, persistent scheduling may be used for new transmissions while dynamic scheduling is used for retransmissions. Semi-persistent scheduling may serve real-time applications such as voice over internet protocol (VoIP) better than the other types of scheduling. When the WTRU buffer is empty, the WTRU may send a signal to release the resource allocation.
In persistent scheduling, the UL resources are granted for repeated transmissions at specific intervals (i.e., one UL grant is valid for a series consecutive UL transmissions). For applications such as VoIP, persistent scheduling may guarantee real time service and save lots of signaling overhead. However, for every UL transmission, a retransmission opportunity has to be scheduled.
Given the numerous options for performing scheduling of UL resources, there a detailed procedure for providing spectrum access to secondary devices is needed.